1. Field of the Invention
The invention relates to an exhaust gas purification device for purifying an exhaust gas discharged from an engine.
2. Description of the Related Art
An exhaust gas purification device is known for purifying an exhaust gas, in particular, nitrogen oxide (NOx) discharged from an engine. For example, the Japanese Unexamined Patent Publication No. 6-74022 discloses an exhaust gas purification device comprising a NOx catalyst which adsorbs NOx and hydrocarbon (HC) on a surface thereof to activate the NOx and HC, and reacts the activated NOx with the activated HC to purify the NOx, and a HC feeding valve for feeding HC to the NOx catalyst.
When the velocity at which the flame is transmitted in the cylinders during the power stroke of the engine (hereinafter, referring to as "flame transmitting velocity") is large, the amount of the NOx generated in the engine is large. Further, when the temperature at which the fuel is burned in the cylinders during the power stroke of the engine (hereinafter, referring to as "fuel burning temperature) is high, the amount of the NOx generated in the engine is large.
On the other hand, an inert gas decreases the flame transmitting velocity. Therefore, when the amount of the inert gas in the intake air is large, the flame transmitting velocity is decreased. Further, the inert gas absorbs the heat. Therefore, when the amount of the inert gas in the intake air is large, the fuel burning temperature is low.
In the exhaust gas purification device disclosed in Japanese Unexamined Patent Publication No. 6-74022, in order to restrict the generation of the NOx in the engine, exhaust gas including inert gas such as CO.sub.2 and H.sub.2 O is introduced into an intake air to decrease the flame transmitting velocity and lower the fuel burning temperature.
Further, in the exhaust gas purification device disclosed in Japanese Unexamined Patent Publication No. 6-74022, in order to prevent the HC from being introduced into the intake air, the exhaust gas to be introduced into the intake air is withdrawn from one of exhaust passages while the HC is fed into the other exhaust passages.
However, since the exhaust strokes in the cylinders are sequentially carried out, there is a possibility that the pressure in the exhaust passages into which the HC is fed becomes higher than that in the exhaust passage from which the exhaust gas to be fed into the intake air is withdrawn. In this case, the HC in the exhaust gas may be introduced into the intake air.
Therefore, the object of the invention is to prevent HC to be fed into a catalyst from being introduced into the intake air.